Secondary batteries are classified into, depending on the shape of battery cases they employ, a cylindrical battery and a prismatic battery in which an electrode assembly is accommodated in a cylindrical or prismatic metal can and a pouch-type battery in which the electrode assembly is accommodated in a pouch-type case made of an aluminum laminate sheet.
The electrode assembly accommodated in the battery case is a power-generating device with a cathode/separator/anode structure that can be charged and discharged. The electrode assemblies are classified into a jelly roll-type structure which is obtained by interposing a separator between long sheet-like cathode and anode to which active materials are coated and then winding the same and a stacked structure in which a plurality of cathodes and anodes with separators interposed therebetween are stacked sequentially. The jelly roll-type electrode assembly is widely used due to their advantages including ease of fabrication and high energy density per weight. The jelly roll-type electrode assembly is commonly employed in cylindrical batteries.
The jelly roll-type electrode assembly undergoes repeated expansion and contraction during charge and discharge of the battery and, as a result, it tends to be deformed. During the charge and discharge, stress is concentrated at the central portion of the electrode assembly to cause the electrodes to penetrate the separator and contact with a metal pin at the center, resulting in internal short circuits. Heat caused by the internal short circuits leads to decomposition of an organic solvent and generation of a gas. The gas increases the internal pressure of the battery, resulting in rupture of the battery. The gas pressure inside the battery may increase when internal short circuits occur due to an external impact.
Attempts have been made to solve the safety problem of batteries. For example, a cap assembly of a cylindrical battery is designed such that a safety vent for exhausting a high-pressure gas, safety devices such as a PTC device for interrupting current at high temperature, a current interrupting device (CID) for interrupting current when the internal pressure of the battery is increased, etc. and a top cap forming a protruding terminal to protect the devices, etc. are fixed together by a sealing gasket. In general, the sealing gasket is designed to support the CID downwardly to prevent floating. However, in such an upward supporting structure, the CID filter may not be fixed sufficiently and floating may occur due to an impact delivered from inside or outside the battery. In particular, when a gas is produced inside the battery, the CID may be floated by the gas and the discharge of the gas may be interrupted. In addition, if the CID filter is welded to the safety vent in the state where it is not fixed sufficiently, assembling processability may be unsatisfactory because it is not easy to control the position of the CID filter and a gas discharge hole of the top cap.
FIG. 1 shows the structure of a cap assembly of a cylindrical secondary battery disclosed in Korean Patent Application Publication No. 2012-0041511. The cap assembly includes a top cap 110, a PTC safety device 120, a safety vent 130, a current interrupting device (CID) 150, a CID gasket 140 and a sealing gasket 160. The safety vent 130 is formed such that its center portion protrudes downward and a notch 131 is formed near the center portion. The CID 150 may be at least partly welded to the lower end of the safety vent 130. Accordingly, the downwardly protruding portion of the safety vent 130 contacts with the CID 150 under normal state. But, when the shape of the safety vent 130 is reversed as the internal pressure is increased due to gas production, the electrical connection between the CID 150 and the safety vent 130 is interrupted. The CID gasket 140 is configured to surround the outer circumference of the CID 150 and the sealing gasket 160 is arranged to surround the edges of the top cap 110, the safety device 120 and the safety vent 130 and support the CID 150 downwardly. Although the CID device is supported by the gasket downwardly, its top or lateral sides are not fixed. Accordingly, it is highly likely that the CID device may float when a gas is produced. Therefore, it is necessary to solve this problem.